When a call is made from a fixed telephone, user equipment (UE) or the like, a sound, which is transmitted to the fixed telephone or the UE of a caller, when the caller has made a call to a preferred destination and receives a telephone can, is referred to as a “Ring Back Tone”.
In a 3G (third generation: third generation mobile communication) network which is an existing public mobile communication network and in which an incoming call from the UE such as a cellular telephone is enabled, a Ring Back Tone unique to each system operator of a telecommunications carrier or the like (SRBT: Special-RBT) is used.
In such an existing 3G network, an MSC (Mobile Switching Center) which is a switching center includes a sound source of the SRBT, and the SRBT is provided from the above described MSC to the UE of the caller.
FIG. 1 shows a processing procedure in which the UE makes the call and in which the SRBT is transmitted to the above described UE in the existing 3G network.
When arbitrary UE makes the call, an MSC of a communication system (hereinafter referred to as “calling side system”) which accommodates the above described UE (hereinafter referred to as “calling UE”) executes an MM Connection Procedure including an authentication process for the calling UE, and a Radio Bearer Setup Procedure such as setting a band used for radio communication with the calling UE.
When the MM Connection Procedure with the calling UE is completed, the MSC of the calling side system transmits an IAM message for requesting that a call be connected to an MSC of a communication system (hereinafter referred to as “called side system”) which accommodates UE (hereinafter referred to as “called UE”) of the call destination.
When the MSC of the called side system receives the IAM message, the MSC of the called side system executes the MM Connection Procedure including the authentication process for the call may be incoming in UE1, which is the transfer destination of call, and the Radio Bearer Setup Procedure such as setting of a band used for radio communication with the call may be incoming in UE1, which is the transfer destination of call, similarly to the calling side system.
When the Radio Bearer Setup Procedure is completed and preparations are made to connect the incoming call, the UE that received the call calls the user, and also transmits an ALERT signal, indicating a start of calling the above described user, to the MSC of the called side system.
When the MSC of the called side system receives the ALERT signal from the called UE, the MSC of the called side system transmits a call progress message (CPG), indicating an occurrence of an event (the user is being called) concerning which the calling side is to be notified, to the MSC of the calling side system.
When the MSC of the calling side system receives the above described call progress message, the MSC of the calling side system transmits the ALERT signal, indicating the start of calling a called side user, to the calling UE. Subsequently, the MSC of the called side system uses the sound source included in the device itself to transmit the SRBT to the calling UE.
Note that a configuration for outputting the SRBT to an IP telephone connected to a fixed network is described, for example, in Patent Document 1. Patent Document 1 describes a technique in which, when a transmission request is received by a call connection controller from a calling side PC, the Ring Back Tone is sent out to the IP telephone in cooperation with the above described calling side PC.
Incidentally, in recent years, a femto cell in which a wired line installed at home, an office or the like is used to connect the UE to a mobile communication core network, has drawn attention.
The femto cell is a radio communication scheme which is assumed to be mainly for indoor radio communication at home, the office or the like, and uses a Femto AP covering a small communication area with a radius in the order of several tens of meters to enable radio communication with the UE. Thereby, various communication services can be provided to the user using the UE, without requiring a maintenance cost for a radio base station, a Radio Network Controller, the MSC and the like which support an existing macro cell.
However, like the existing 3G network shown in FIG. 1 and a system configuration described in the above described Patent Document 1, the communication system including the Femto AP of the related art is not configured assuming that the SRBT is transmitted to the calling UE accommodated in the Femto AP.